28 CELL-FORMATION. [BOOK i. 



that the beginning of a cell may not be of this nature. 

 Such a theory is not incompatible with the evidence revealed 

 by the microscope as soon as the real structure of cells 

 becomes visible ; for there is a period anterior to the acqui- 

 sition of such size as optical instruments can examine, in 

 which we know nothing of what is taking place in the matter 

 from which cells are derived. This is manifest upon the 

 examination of cambium. It must, however, be a subject of 

 hypothesis, because direct evidence upon the point is, from 

 its nature, unattainable. It is no objection to this hypo- 

 thesis that each cell has its own independent sides, because 

 the same fact attends the extrication of gas in viscous fluids, 

 and the intercellular matter may in that case be regarded as 

 a remaining portion of mucus in which no gaseous matter 

 has been formed. 



B. New cells are produced by old cells , on their outside, as 

 new branches are produced by old ones. Amici says that the 

 new tubes of Chara appear like young buds, from the points 

 or axils of pre-existing tubes, an observation which has been 

 confirmed by Slack. It has been stated by Mirbel that the 

 same thing occurs in the case of Marchantia polymorpha. 

 That learned botanist, in the course of his inquiries into the 

 structure of this plant, found that in all cases one tube or 

 utricle generated another externally, so that sometimes the 

 membranes of newly-formed tissue had the appearance of 

 knotted or branched cords. He satisfied himself that new 

 parts are formed by the generative power of the first utricle, 

 which spontaneously engenders on its surface others endowed 

 with the same property. The amylaceous vesicles of malt in 

 a state of fermentation manifestly produce new vesicles from 

 their sides externally; and Turpin asserts that they also 

 contain molecules, which are the rudiments of other cells. 



C. New cells are formed in consequence of the internal sub- 

 division of an old cell by a plait, fold, or septum, projecting into 

 its cavity. This, which is named " merismatic," or " fissi- 

 parous" development, is universal among pollen (which see 

 hereafter), and has been regarded as the general mode of cell 

 multiplication. The following observations by Mr. Thwaites 

 upon certain Algals detail the appearances which lead to 



